It has been known that a power conversion circuit is connected between a direct current power source and a motor and performs a conversion between the power at the side of the direct power current source and the side of the motor in response to the powering operation and regenerative operation carried out by the motor. When the motor is rotated by the external force, the regenerative current flows from the motor to the power source.
Accordingly, the magnitude of the power source current flowing into the power conversion circuit from the direct current power source affects the balance of payments for power consumption as comparing with the other systems which are using the same direct current power source at the time of driving the motor. For the motor driving system requiring larger power consumption in particular, the restriction of drawing the power source current is required. For example, the apparatus disclosed in the Patent Document 1 restricts the current command value based on the power source voltage and the motor rotational angular speed and restricts power drew from the direct current power source.
The influence caused by a regenerative current changes due to the allowable recharging power of the direct current power source at the time of the regenerative operation carried out by the motor. When the allowable recharging power is relatively lower, the voltage at the side of the power conversion circuit rises due to the regenerative current and may damage, for example, switching elements for configuring the power conversion circuit. The apparatus disclosed, for example, in the Patent Document 2 monitors the voltage at the side of the inverter device (i.e., the power conversion circuit) and stops the driving of the inverter when detecting an abnormal value.
In the configuration for restricting the current command value in response to the power source voltage or the motor rotational angular speed such as the apparatus disclosed in the Patent Document 1, since the relation between the motor rotational angular speed and current restriction value may vary when the motor output properties change, it is possible that the apparatus cannot restrict to a predetermined current (i.e., power) when, for example, the wiring resistance value at the side of the direct current power source changes.
In addition, when the inverter stops driving in case of detecting an abnormal value, the assistive function for lightening the steering force exerted by a driver may not work in, for example, the vehicular electric power steering apparatus such as the apparatus disclosed in the Patent Document 2. As a result, it brings uncomfortable feeling to the driver and inconvenience in steering a vehicle.    [Patent Document 1] JP 2008-049910-A (Corresponding to US 2008/0047776 A1)    [Patent Document 2] JP 2009-136111-A